1. Field of the Invention
The present invention relates to a magnetic recording or reproducing apparatus for recording information on a magnetic tape or for reproducing information recorded on the magnetic tape using a rotary magnetic head.
2. Description of the Related Art
The development of video equipment such as video tape recorders (VTR) has been remarkable in recent years. In particular, in the field of VTRs, there has been an increasing demand for a reduction in size and weight thereof, as VTRs have grown in popularity.
Generally, the above-described VTR is of a helical scanning type which employs a rotary head drum. Compared with the other components of the helical scanning mechanism, the rotary head drum is large in volume and weight. Hence, reduction in the size of the rotary head drum is very important to achieving any reduction in the size and weight of the VTR itself.
A technique for reducing the size of the rotary head drum has been known, in which a magnetic recording pattern is obtained on a magnetic tape rotated at a speed of 2700 rpm using four heads. The magnetic recording pattern obtained in this technique is the same as that obtained when a rotary head drum having two heads on the periphery thereof is rotated for the NTSC system at a speed of 1800 rpm. The two heads are precisely 180.degree. apart from each other in the latter technique.
Where D.sub.1 is the diameter of the drum with two heads, the drum with four heads has a diameter D.sub.2 calculated by the following equation: EQU D.sub.2 =2/3.times.D.sub.1 . . . (1)
Thus, the diameter of the drum for the four head system can be reduced by 33% from that of the drum for the two head system.
In addition to the above-described head systems, a rotary head drum that can be employed for an 8 mm video tape recorder which uses a magnetic recording medium (a magnetic type) having a width of 8 mm is also known. The diameter of this rotary head drum is far smaller than that for the four head system.
FIG. 1 shows a system for running a tape relative to the rotary head drum which is employed in a conventional magnetic recording or reproducing apparatus which may be a VTR. A rotary upper drum 1 which forms a rotary head drum has four magnetic heads 2a to 2d on the periphery. The magnetic heads 2a to 2d protrude slightly so as to press into the tape surface. The rotary upper drum 1 with the four magnetic heads is rotated at high speed by a known means (not shown). A magnetic tape 3 which is pulled around the rotary head drum by a known tape loading mechanism (not shown) travels in the direction indicated by the arrows. The tape 3 is in contact with the rotary head drum at a predetermined angle.
In the apparatus which employs the above-described rotary head drum, the rotary upper drum 1 which is the rotary head drum is rotated at extremely high speed compared with that at which the magnetic tape 3 is run, and the magnetic tape 3 is moved relative to the rotary upper drum 1 by means of an air film layer which is generated between the magnetic tape 3 and the rotary upper drum 1.
However, the conventional magnetic recording or reproducing apparatus of the above-described type suffers from a problem in that employment of a rotary head drum having a reduced diameter causes a reduction in the thickness of the air film layer generated in the gap between the magnetic tape and the rotary drum.
More specifically, the thickness AT of the air film layer is calculated as follows: EQU AT.alpha.R(6.mu.V/T).sup.2/3. . . (2)
where AT is the thickness of the air film layer, T is the tape tension, .mu. is the viscosity resistance, R is the radius of the drum, and V is the relative velocity. As is clear from the above-described equation, the thickness AT is proportional to the relative velocity V to the two-thirds power. In consequence, the thickness of the air film layer generated in an apparatus such as an 8 mm video tape recorder which employs a very small drum is particularly small. It is also to be noted that this tendency is greater with a rotary drum having four heads than with a drum having two heads.
As a result, if dust or dirt attaches to the surface of the rotary drum or of the tape, the air film layer is easily discontinued, deteriorating the stability with which the tape travels. A reduction in the thickness of the air film layer changes a state wherein the magnetic tape is in contact with the rotary drum where it leaves the rotary drum and where the tape tension is high, and this facilitates attachment of dirt to the rotary drum. It also increases the possibility of the magnetic tape being damaged and of deterioration in picture quality due to the generation of swinging of the screen.
Further, in the conventional apparatus shown in FIG. 1, the magnetic tape vibrates every time each of the heads makes contact with and leaves it while it is travelling. The tape also vibrates toward and away from the rotary head drum. However, when the intervals of the adjacent magnetic heads 2a to 2d are small, it is difficult for vibrations of the magnetic tape to be sufficiently damped and removed, making the travel of the tape unstable and degrading the head touch. These vibrations cause jitter or wow flutter during recording or reproduction, which adversely affects the image quality by generating time-base error, noise or swinging of the screen.